2.1 Field of the Invention
The present invention relates to electrochemical batteries, and, more specifically, to lithium-metal batteries. More particularly, the present invention relates to methods and compositions that enhance the cycling efficiency of lithium-metal batteries, and, especially, lithium-active sulfur batteries. The present invention has applications in the fields of electrochemistry and battery technology.
2.2 The Related Art
Lithium battery technology continues to be an attractive option for providing light-weight, yet powerful energy sources. Lithium-sulfur secondary batteries are especially well suited to continuing market demands for more powerful and highly portable electronic devices. Examples of such batteries include those disclosed by De Jonghe, et al., in U.S. Pat. Nos. 4,833,048 and 4,917,974; and by Visco, et al., in U.S. Pat. No. 5,162,175. Nevertheless, the batteries described in these, and other references, have serious limitations (Rauh 1979; De Gott 1986). In particular, batteries using sulfur or polysulfide electrodes in combination with lithium, such as the Li.sub.2 S.sub.x batteries described by Peled and Yamin in U.S. Pat. No. 4,410,609, have suffered from poor cycling efficiencies (Rauh 1989).
Many of these difficulties are addressed by the batteries described in U.S. Pat. Nos. 5,523,179 and 5,532,077, both to Chu, each of which is incorporated herein by reference in its entirety and for all purposes. Briefly, the '179 and '077 patents describe solid-state batteries that comprise a lithium electrode in combination with an active sulfur-containing electrode. An "active sulfur" electrode is an electrode comprising elemental sulfur, or sulfur in an oxidation state such that the sulfur would be in its elemental state if the electrode was fully charged. The technology described in these patents is an important advance in lithium battery technology, in particular by describing batteries having large energy densities and good cycling performance.
Nevertheless, the challenge of providing improved batteries, and especially batteries having cycling efficiencies of better than 70%, remains. In particular, the cycling efficiency of lithium-sulfur batteries is limited by the slow degradation of the lithium electrode surface arising from the formation of dendritic and/or high surface area "mossy lithium"--lithium-sulfur complexes that are detached from the lithium electrode and float freely in the electrolyte. These phenomena prevent high cycling efficiencies by steadily degrading the lithium electrode surface over successive rechargings. To compensate for this loss, extra lithium must be provided for the lithium electrode increasing the cost and weight of the battery. The use of additional metals also increases the burden of disposing the battery as additional toxic materials must be processed. Mossy lithium can also present a fire hazard by creating fine particles of lithium metal that can ignite on contact with air.
Various attempts have been made to provide lithium batteries having high cycling efficiencies with some success. One type of lithium battery, commonly used for military applications, comprises a lithium-sulfur dioxide (Li--SO.sub.2) battery to which LiAlCl.sub.4.3SO.sub.2 has been used as an inorganic electrolyte. The cycling efficiencies of such batteries have been demonstrated to be as high as 95% (Dey 1989; Dunger, 1993). However, Li--SO.sub.2 batteries are not a viable alternative for the civilian commercial market. Thus, there remains a need for commercially viable batteries having high cycling efficiencies. The present invention meets these and other such needs.